This invention relates to well drilling and cementing operations.
The general procedure of drilling an oil or gas well includes drilling a borehole using a drilling fluid. Subsequent to drilling the borehole, casing is run into the well preparatory to placing a cement in the annulus between the outside of the casing and the borehole wall. In order to obtain a good cementing job it is necessary to displace substantially all of the drilling fluid or mud in the annulus with cement.
This necessity arises from the fact that undisplaced mud and filter cake become a source of unsuccessful cement jobs since drilling fluids and cements are usually incompatible. Undisplaced drilling fluid is not a reliable sealant to isolate zones in the annulus and is not a good structural support for the casing. Thus, most water-based muds will adversely change the setting time and compressive strength of the cement. Also, most cements will flocculate and thicken most water-based muds. As the cement is pumped out of the bottom of the casing and up the annulus, it may form flow channels through blocking sections of flocculated drilling mud. In addition, undisplaced filter cake can prevent cement from bonding to the formation and becomes a source of the flow channels.
The drilling industry has sought to overcome these problems by using a variety of techniques to displace the drilling fluid with cement, e.g., utilization of turbulent flow regimes, casing movement (reciprocal/rotation), casing equipment (centralizers, flow diverters, and mud scratchers), spacers and special wash fluids while cementing, but these have had limited success.
Even wash fluids are incapable of removing all of the filter cake since it is at least partially adhered rather tightly to the wall of the wellbore. In addition, wellbores are frequently irregularly shaped with large sections which become filled with drilling fluid. Fluid occupying such irregularly shaped or enlarged sections is often out of the main fluid flow stream in the borehole and is therefore difficult to remove.
Even greater cementing difficulties are encountered with extended reach boreholes and slim hole wells with major problems arising in connection with running casings, displacement of mud and centralization of the casing due to the restricted annulus. When a poor cementing job results due to these problems it may be necessary to perforate the casing and squeeze cement under high pressure through the perforations into the annulus and try to fill the zones that were not properly cemented initially. Even this may not be successful and can result in eventual abandonment of the hole. For one thing, the drilling mud and filter cake resulting therefrom, and conventional cements are simply not compatible.
In addition, many formations have high calcium ion content which creates problems with drilling fluids.
Finally, disposal of what drilling fluid can be successfully removed from a borehole presents its own problems.
Hence, incompatibility of drilling and cementing fluids remains a problem. Even if, through extraordinary efforts, the compatibility problems are mitigated, a Portland cement composition is rigid and not tolerant of flexing. Also, while generally viewed as a strong, hard substance, in fact Portland cement has limited tensile strength.